Molded plastic products which have substantial three-dimensional or high-relief surfaces can be formed from numerous plastics using injection molding. Injection molding dies for high-relief products, however, are often quite expensive, making their use difficult to justify unless very large production runs are contemplated.
Rotational casting of plastic products is also a well-known plastic forming technique. The plastic compounds which are suitable for rotational casting, however, tend not to have the desired strength and impact resistance which can be achieved in connection with injection molded plastics.
Attempts have been made to combine injection molded and rotationally molded plastics into useful products having three-dimensional design features or ornamentation. Coffee cups have been manufactured, for example, which are a combination of an injection molded plastic central cup or core member, around which a rotationally molded decorative sleeve is secured. Such sleeves have three-dimensional design features on the exterior thereof, which are considerably less costly to rotationally mold than would be the case if the same design were injection molded as a part of the injection molded central cup.
Prior composite injection molded and rotationally molded products, however, have been found to have several substantial disadvantages. First, the three-dimensional design features add a depth dimension or relief which was relatively low. Second, mounting of the rotationally sleeves on the injection molded central cup members has required heating of the rotationally molded sleeve, with the result that once mounted on the cup, the sleeve shrinks down into direct contact with the cup member. This produces a product in which the sleeve is relatively hard and inflexible as the result of being supported directly by the hard and inflexible injection molded cup or core. Moreover, since the sleeve is in direct contact with the core over a substantial area, heat transfer through the combined walls is more rapid, which for products such as coffee cups is undesirable.